The present invention relates to a connecting structure, in which a rotary shaft, such as a valve shaft or the like, supporting, for example, a throttle valve, and a metal plate constituting a gear or the like are fixedly connected to each other so as to be able to rotate together, and a connecting method therefor.
For example, JP-A-11-192567 (pages 1 to 8, and FIGS. 1 to 11) describes, as a known technique of joining a rotary shaft and a metal plate together, a method, in which a large-diameter shaft portion (central shaft) and a small-diameter shaft portion (projecting shaft portion) are connected to each other with a step therebetween so as to define a shoulder surface orthogonal to a central axis, a circular fitting hole fitted therein the small-diameter shaft portion is formed on the metal plate, the fitting hole of the metal plate is then fitted onto the small-diameter shaft portion, and then the metal plate and the shoulder surface are joined together by means of laser welding.
JP-A-9-250627 (pages 1 to 10, and FIGS. 1 to 7) describes a connecting structure between a shaft and a metal plate, in which diameter-enlarging streaks are formed on an end port of a rotary shaft (drive shaft) to project from an outer peripheral surface of the shaft while, a shaft mounting hole is formed on a metal plate (pulley), the shaft mounting hole is press-fitted onto the end port of the rotary shaft so as to cause plastic deformation by the diameter enlarging streaks while an outer periphery of the tip end of the rotary shaft is reinforcement-welded to the shaft mounted hole.
Further, JP-A-5-200475 (pages 1 to 4, and FIGS. 1 to 11) describes a method of manufacturing a rotary shaft, in which a molding die is used to form swaged streaks (raised portions) on an outer periphery of the rotary shaft to enable a pulley, a rotor, a commutator, a fan, etc. to be fixedly connected thereto.
By the way, in the case where the connecting structure described above is applied for joining a rotary shaft to a metal plate, such as gears, pulleys, sprocket wheels, etc., in order to transmit a rotational drive force of a motor or the like to the rotary shaft, for example, joining between a valve shaft (rotary shaft) in an electrically controlled throttle device, to which a throttle valve is to be mounted, and a throttle gear (metal plate) joined to the valve shaft in order to permit transmission of a rotational drive force of a motor to the valve shaft, the metal plate is always exerted by a force in a direction of shaft length (direction along an axis of rotation) in addition to a force in a direction of rotation and is frequently applied with a repeated load so that it is necessary to ensure a sufficient joining strength between the rotary shaft and the metal plate. Thus it is demanded to ensure coaxiality and orthogonality between the both elements with a high degree of accuracy.
With the connecting method described in the JP-A-11-192567, however, the metal plate and the shoulder surface on the shaft are joined together by means of laser welding, so that inclination of the metal plate relative to the shaft is restricted. Thus the method cannot be applied other than a very thin (thickness is 1 mm or less) metal plate although orthogonality is achieved to some extent. Also, there is caused a problem that a gap (clearance) is ordinarily defined between the small-diameter shaft portion (projecting shaft portion) and the fitting hole to result in eccentricity of the metal plate (or its fitting hole) relative to the shaft, so that satisfactory coaxiality cannot be obtained. In this case, it may decrease the gap in order to improve coaxiality, but it is necessary to control an outside diameter of the rotary shaft (its small-diameter shaft portion) and an inside diameter of the hole of the metal plate with high accuracy, resulting in an increase in cost.
On the other hand, with the connecting structures described in JP-A-9-250627 and JP-A-5-200475, when the hole (the shaft mounted hole, the mount hole) on the metal plate (pulley or the like) is press-fitted onto the rotary shaft, the diameter enlarging streaks or the swaged streaks bite into the hole to subject the same to plastic deformation (scraped), so that a gap defined between the rotary shaft and the hole is substantially absorbed whereby eccentricity of the metal plate (its hole) relative to the shaft is restricted and sufficient coaxiality can be obtained. However, in the case where a sheet (having a thickness of, for example, 3 mm or less) having a large wall thickness portion such as a boss or the like is used as the metal plate, this connecting structures cannot be practically used because the metal plate tends to incline relative to the rotary shaft (orthogonality can hardly be obtained) unless the portion around the hole in the metal plate is fairly large in thickness (length in a direction along the axis of rotation).
The present invention is devised in order to solve the problems described above and an object of the present invention is to provide a connecting structure between a rotary shaft and a metal plate and a method of connecting them together, in which the metal plate can be joined to the rotary shaft with a high degree of accuracy and high strength at a low cost without generating eccentricity, inclination, or the like as far as possible.
Further object of the invention is to provide an electrically controlled throttle device which is reasonably applied thereto with the connecting structure.